What is Focus

December 12, 2020

Focusing (from Latin: focus - hearth, fire) the process of combining the optical image of the object being photographed with the plane of the light receiver.

Due to the imperfection of any optical systems, in particular, shooting lenses, instead of an image of an ideal point, a spot (circle) of scattering is obtained, the shape and dimensions of which depend on the position of the plane on which the image given by the lens is obtained. By moving this plane along the optical axis, you can get two of its positions corresponding to either the highest resolution of small details (highest resolution), or the highest sharpness of the contours of their images (borderline unsharpness).

As an optimal position image plane, given by a particular lens, its position is taken at which the most advantageous combination of resolution in images of small details with their sharpness and distinctness is obtained. Such a plane is called the “best fit plane”. In the process of focusing, it is desirable to combine this particular plane of the optical image with the plane of the photodetector.

Focusing

What does focus depend on?

SLRs and cameras that use phase focusing sensors have such a thing as "permissible blur spot". Relatively speaking, this is the range, once in which, the camera will consider focusing completed. At the same time, the picture on the display will be sharp, and then when viewed on the monitor it will turn out that it is sharp where we do not need it. And it's terrible. Of course, this is also superimposed on gaps in the focusing mechanism of the lens and camera. And on a similar device, this can occur more often than on a new one. How to prevent this disease? Very simple - we take several shots with refocusing at the same point. Usually this helps to get 10 frames from a series, 5-7 pieces are acceptable sharp for our eyes.

Tochka AF v VIDOISKATELE

Smartphone and camera cameras, which use a contrast or hybrid phase-contrast focusing method, do not suffer from such a disease. In DSLRs, it is turned on in LiveView mode, and in mirrorless cameras and smartphones it is used by default. This method is more accurate but slower. Although in modern cameras it has already caught up with the phase focusing in terms of speed. In general, in both cases, the camera will focus on the contrast in the focus point / area.

In fact, DSLRs usually have the most sensitive central focus point, and allow you to focus even in poor lighting. The side dots are sensitive, but not as good.

Vybor tochek AF

It is recommended to use the entire group of points when it is necessary to enable object tracking. In this case, it is more important for us to get some kind of sharp image than not to get it at all. Although, some cameras do this in a completely unpredictable way, focusing as if on high on anything but what is needed.

At the same time, in the viewfinder of a SLR camera, in addition to confirming focus, you can tense up and see the depth of field. Mirrorless cameras, on the other hand, have such things as focus peaking, including which we can see sharpness both in the viewfinder and on the display. Focus-peaking emphasizes the sharp contours of objects with additional color so that it is clear without magnifying the picture where we are focused. A very useful thing when focusing both manually and in auto mode with fine-tuning by hand.

Dependence of focusing on the design of the autofocus drive of the lens

There are more precise mechanisms, such as the USM in Canon, there are less accurate ones, such as (now considered archaic), a traditional gear drive, which rolls the glass around the insides. In the case of using such an old mechanism, the accuracy depends on the size of the drive gears, as well as its wear. Typical problematic lenses in this regard:

And other junk of the ultra-lower price segment. In the case of increased wear, the camera can, of course, get into focus, and even issue confirmation, but the backlash in the mechanism, even half a millimeter at the output, gives jokes on the focus spread.

TRADICIONNYJ MEHANIZM AVTOFOKUSIROVKI

The moments with overshoots and undershoots are especially obvious and infuriate when we shoot (especially after zooms) on lenses with a fixed focal length and wide apertures like f1.8. On the open, and so the depth of field is small, and then there are all sorts of features. In general, now, of course, the circumstances with this matter are much better than before, in cameras with a hybrid focusing system, however, in order to save money, some manufacturers are still in no hurry to bring the bottoms up to the level of top lenses.

Dependence of focus on the size of the sensors

As it turns out, the accuracy of AF when focusing still depends on the size of the sensors (we are considering DSLRs) that are responsible for focus accuracy. And the path of light to these sensors lies not only through the lens, but also through the path of the camera, in which there is usually an additional mirror behind the main mirror, which sends part of the light to these sensors. On cameras with large matrices, the sensor area is significantly increased compared to cameras in the lower price segment.

Dependence of focus on the lens scheme

When designing optics, engineers face challenges that require trade-offs. There is such a thing as "spherical lens error and its correction." This is when you kind of unscrewed it, as expected, got sharpness on the open, and then closed the aperture before shooting, iiii, at the exit there was a marriage. So, in optics, focusing accuracy also depends on whether this same spherical lens error is corrected or not. Let's put this jamb of optics on the rest of the dependencies, and we get .... That at a certain moment, focusing, as you don’t adjust the camera and the lens, does not appear immediately after pressing the button.
Lens schemes usually consist of several (tens) elements. In the 60s, such a concept as “floating elements” was introduced in order to maximally correct possible jambs of the resulting image. Lenses have a converging part and a diverging part. These parts are combined with each other using different types of glasses with different refractions. Why is it necessary to correct? The collecting part gives a terrible quality at the input, and usually collects which lens? That's right, curved out. She obviously collects more than necessary, and the rest of the scheme is trying to compensate for what the front end has collected, and not always successfully. Have you seen the photos from the monocle? Well, a true monocle is a converging lens, and that's it. Not a very high quality picture.

Dependence of focusing on ambient temperature

The accuracy of the autofocus and focus mechanisms, in principle, is affected by the ambient temperature. There are gaps in the mechanisms, and these gaps vary in varying degrees from the natural change in the state of aggregation of the substance - the expansion of the gaps reduces and increases the accuracy in theory, and the additional compression (in the cold) increases the gaps.

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